The present invention relates to intracranial pressure measurement. More particularly, the invention relates to a method wherein artifacts such as may result from patient movements may be reduced and/or eliminated from sonomicrometer or similarly obtained intracranial pressure measurements without compromise of the techniques"" non-invasive character.
It is known that a relative measure of intracranial pressure may be non-invasively obtained by measuring changes in a patient""s skull diameter with a sonomicrometer or similar volumetric measurement type device. According to the known method, changes in the patient""s skull diameter are directly related to changes in the patient""s intracranial pressure. Because, however, the measured changes in skull diameter are extraordinarily small, indications of intracranial pressure obtained through this method are highly subject to vibration and/or slow drift artifacts. While vibration artifacts, such as may result from touching of the patient, may generally be removed from the estimate through simple low pass filtering, this is not the case for slow drift artifacts, such as may result from patient movements and the like. Slow drift artifacts are generally indistinguishable in the frequency domain from changes in intracranial pressure caused by edema, infectious process, tumor growth or bleeding. As a result, slow drift artifacts are generally very difficult to remove through standard filtering techniques.
It is therefore a primary object of the present invention to improve over the prior art by providing a method whereby slow drift type artifacts may be eliminated from the sonomicrometer obtained signals, thereby improving the quality of intracranial pressure measurement data derived therefrom. Although those of ordinary skill in the art will recognize that one such means for accomplishing this objective involves securing the sonomicrometer to the patient""s skull with bone screws or the like, such a solution completely destroys the sonomicrometer""s non-invasive character. As a result, it is a further object of the present invention to meet the primary object without sacrifice of the overall technique""s non-invasive character.
In accordance with the foregoing objects, the present inventionxe2x80x94a method for artifact reduction in sonomicrometer obtained intracranial pressure measurementsxe2x80x94generally comprises isolating a component of a sonomicrometer waveform attributable solely to changes in intracranial volume by using a neural network or other nonlinear engine to extract a heartbeat component from the sonomicrometer output. Because the heartbeat is so characteristic, no actual measurement of the heartbeat as the forcing function is required to isolate the resulting changes in distance from the artifact induced changes in distance. The neural network can then be utilized to directly map measured changes in skull distance over time to changes in intracranial pressure over a volume change, the inverse of compliance.
According to the present invention a mapping from the sonomicrometer obtained distance measurements to an index of the mean intracranial pressure is obtained by using a neural network to examine only those components of the distance measurements corresponding to a known forcing function. According to the preferred implementation function, the known forcing function is chosen to be the heart beat waveformxe2x80x94a well-documented and highly characteristic waveform. By examining only those distance changes associated with the heart beat forcing function, artifacts associated with patient movement and the like are eliminated notwithstanding the fact that they may have fundamental frequencies corresponding to changes in intracranial pressure due to edema, infectious process, tumor growth and especially bleeding.
Finally, many other features, objects and advantages of the present invention will be apparent to those of ordinary skill in the relevant arts, especially in light of the foregoing discussions and the following drawings and exemplary detailed description.